| The belt of water and wind erosion crisscross is the region with most serious soil and water loss on the northern of Loess Plateau in China, and also is the region which need to carry out series of ecological environment construction. How to quickly and efficiently control of soil and water loss, and improve the ecological environment through the wegetation restoration, has become the most significant facor to restrict the economic and social sustainable development on this region. Over the past decades years, in order to control soil and water loss and reconstruct ecological environment, the Chinese Government has carried out a large number of measurements with vegetation restoration, which have a strong influence on the pattern of land use and ecological environment on the Loess Plateau. However, there are new environmental problems occurred on many regions, such as, the artificial vegetation and land degradation, which aggravated the conflict (?)etween water resources and vegetation restoration. The main cause of this phenomenon is ack of deep understanding of the mutual relationships among the "soil water-vegetation-climate condition", and caused that the planting density and primary productivity of artificial vegetation is always greater than the carrying capacity of water resourcesin the rocess of vegetation restoration. Therefore, it is necessary and crucial to study about the water movement and transformation rule in "soil-vegetation-atmosphere system", and (?)he soil water carrying capacity for vegetation in typical small watershed on the belt of water and wind erosion crisscross. These studies can reveal the water consumption characteristics for representative vegetation and the soil water dynamics in deep soil rofiles, and these are significant for the rational utilization of soil water resources and the sustainable development of reconstructed ecosystem. The study was conducted on a typical small watershed (Liu Daogou watershed) on the belt of water and wind erosion crisscross, and we chose typical shrub and grass vegetation on plot and slope scale as the research (?)bject to carry out the experimental study. The main conclusions of this study are as fllows:1) The soil water stortage (SWS) in 0-1.0,1.0-2.0,2.0-3.0, and 3.0-4.0 m layers were collected using neutron probes at 11 sites in each of four land-use types:cropland, fallow land, grassland, and shrubland. This study tested and validated the feasibility of estimating mean SWS over multiple years by the SWS at selected locations. The most time-stable locations (MTSLs) for the various layers and the location at mid-slope for each land use wereselected on 20 sampling occasions during a calibration period from July 2004 to December 2005. Avalidation data sets from January 2006 to October 2013 was used to test the length of time the estimatesof mean SWS remained valid. The SWSs in grassland and shrubland decreased with plant growth, and the temporalvariations were larger in grassland and shrubland than in cropland and fallow land. The temporal stability of the SWSs was high for all soil layers in each land uses, with the rank correlations over the threshold of significance (a= 0.05) over 10 years. The degree of temporal stability of SWSs was ranked as cropland> fallow land> grassland> shrubland, and the temporalstability of SWSs in grassland and shrubland L decreased with increasing lengths of observation period, as indicated bylower mean Spearman’s correlations for all soil layers. The MTSLs selected from the calibration periodcould accurately estimate mean SWSs for diverse layers under four land uses with estimation errors lessthan 10% over eight years. The study verified that a single location at mid-slope of each land use could besampled in order to reduce the required number of samples and save time and labor while maintaining a high accuracy of prediction over multiple years.2) Soil water is a critical factor for vegetation growth and distribution, however, precipitation and vegetation actively affect soil water dynamics. Understanding the effects of annual precipitation patterns and land uses on soil water dynamics is critical to provide guidance for soil water management and land use planning in the Northern Loess Plateau. The profile characteristics and temporal dynamics of soil moisture in 0-400 cm were examined in four land uses, including shrubland (Caragana korshinskiiKom.), grassland (Medicago sativa), fallow land and cropland under four annual precipitation patterns. Precipitation and land use joint control the spatial and temporal changes and distribution of soil water in deep soil profile. Precipitation mainly affects the spatiotemporal distribution of soil water in 0-1.0 m profile, and the effect of land use on soil water was reflected in the 0-4.0 m profile. Under different annual precipitation patterns, soil water in cropland and fallow land were significantly higher than those in shurbland and grassland, following the sequence of cropland> fallow land> grassland> shrubland in 0-4.0 m; and the difference of soil water in land uses planting with alfalfa and caragana were small. Compared to cropland, the depths of soil water depletion for caragana, alfalfa and grass in fallow land surpassed 4.0 m among four study years. Compared with cropland, caragana, alfalfa and grass in fallow land of more than 4.0 m. The depth of precipitation infiltration and the thickness of dry layer were always differnent under the four annual precipitation patterns. Under all annual precipitation patterns, there were no dried soil layers in cropland and fallow land, however, desiccated soil layers formed in shrub land and grassland and all deeper than 400 cm. Under shurbland and grassland, the depth of precipitation infilitration is always less than 1.0 m in normal, dry and wet year, and even in extreme wet year, the maximum infiltration depth is less than 2.0 m. Therefore, under the condition of natural percipitation, once the dried soil layer formed in the deep soil profile under land uses with planting artificial vegetation, soil water will be hard to get supplies and improving.3) We analysed the response of soil water and plant growth to planting density for two dominant shrubs(Caragana korshinskii Kom. and Salix psammophila) in this area buy filed observation. The plant density has significant influences on the growth of vegetation and the spatial-temproal distribution of soil water. Plant height, branch diameter and dry biomass of caragana and salix were all decrease as the plant density increased within a certain range. However, when the density increased to a certain value, plant height, branch diameter and dry biomass of caragana and salix showed a trend of increase; Soil water content and water storage under land of caragana and salix showed a decreasing trend with the increase of plant density. With the plant density increasing, the variation range of soil water dynamics was also decreased, and the degree of soil desiccation was also tend to be more serious.To make sure that the precipitation can recharge soil water and make dry layers could be restored, we recommend the optimal planting density of caragana and salix were 9000 plants/hm2 and 8500 plants/hm2, respectively; and the biggest planting density were 14000 plants/hm2 and 11000 plants/hm2, respectively.4) The plant ages have significant impacts on vegetation growth, soil water variations, and soil water consumption. Soil water for 2-12-year old caragana and 1-19-year old alfalfa declined along with the extension of plant ages. Planting caragana and alfalfa for many years could cause soil decissiation due to large water consumption. During the young period, water consumption was larger and soil water rapidly reduced to the stable soil water content, and the degree of soil desiccation aggravated along with the increase of plant ages. The dried layers in deep soil were difficult to recover after it occurred. The formation rate and thickness of dried soil layer were related to vegetation types. The dried soil layers began to appear below 100 cm when alfalfa was 5 years old, however, the permanent dry layer below 100 cm formed since 6 years old of caragana. Thus, we recommend the optimal survive ages for alfalfa and caragana in the region was 5 years and 6 years old respectively, and this corresponding largest biomass was 1980 kg/hm2 and 5050 kg/hm2, respectively.5) The one-dimensional Simultaneous Heat and Water Transfer (SHAW) model was calibriated and validated using the field measured data, including soil water content in 0-4.0 m soil profile, soil physicial paramters, plant parameters and climate dats in our study watershed. Soil and plant parameters required by the SHAW model were calibrated and validated with meteorological and soil-water data from 2004-2005 and 2012, respectively. The data from the calibration and verification trials for soil water content were significantly linearly correlated based on a 95% confidence level, which inftered that the simulated SMs thus generally agreed well with the measured values in the two plots. During the period of calibration, the ME, RMSE and RMAE for SM in the 1.0-4.0 m profile were 0.34%,1.06%, and 3.81% for caragana and 0.17%,0.88%, and 3.12% for alfalfa, respectively, indicating that the performance of SHAW for the calibration period was better for alfalfa than for caragana. Compared with simulated results in calibration stage, the model precision was lower during the validation period, with RMSEs of 5.71 and 1.14%, but the error ranges were consistent with those reported in other modeling studies. These results showed that SHAW model was thus sufficiently accurate for simulating soil water dynamics in our study area.6) The conflict between soil desiccation and the sustainable development of revegetation is increasingly important on the Loess Plateau in China. Quantitative guidelines for the selection of plant species, optimal density or biomass, and appropriate management for vegetative restoration are required to address this conflict. Caragana and alfalfa, two typical species for vegetative restoration in the water-wind erosion crisscross region of the Loess Plateau, were selected to study the optimal plant age and corresponding biomass based on soil water balance using the SHAW model after calibriation and validation to simulate soil water content variatuions with plant ages under a representative normal year. The simulations indicated that soil water decreased within 1.0-4.0 m profiles and that the depth of water depletion deepened with plant growth after vegetative restoration. Dry soil layers began to develop below 1.0 m after five years for caragana and after three years for alfalfa. The optimal ages of the caragana and alfalfa in the study area were thus five and three years, respectively, and the corresponding maximum biomasses were 4800 kg/hm2 and 1200 kg/hm2, respectively.This study showed that although the land use types and precipitation together effected he spatial and temporal variations of soil water, the SWSs for all soil layers in each land (?)ses were still maintian strong temporal stability during a long period. Different plant densitis and plant ages had significant impacts on soil water dynamics and plant growth. soil water always declined along with the increase of plant density and plant age. Under he condition of natural percipitation, the suitable planting densities of caragana and salix were 9000 plants/hm2 and 8500 plants/hm2, respectively; and the biggest planting density were 14000 plants/hm2 and 11000 plants/hm2, respectively. The optimal survive ages for alfalfa and caragana in the region was 5 years and 6 years old respectively, and their corresponding largest biomass was 1980 kg/hm2 and 5050 kg/hm2, respectively. The simulation studies using SHAW model showed that the SHAW model was suitable for simulating soil water dynamics in our study area, and the optimal ages of the caragana and alfalfa in the study area were thus five and three years, respectively, the corresponding naximum biomasses were 4800 kg/hm2 and 1200 kg/hm2, respectively. These results rovide useful information for designing appropriate practices of vegetative restoration to attain sustainable ecological and economic benefits on the Loess Plateau. |
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